Question: $ C = \left[\begin{array}{rr}2 & 0 \\ 1 & 4 \\ 5 & -1\end{array}\right]$ $ F = \left[\begin{array}{rr}2 & -1 \\ 4 & -1\end{array}\right]$ What is $ C F$ ?
Answer: Because $ C$ has dimensions $(3\times2)$ and $ F$ has dimensions $(2\times2)$ , the answer matrix will have dimensions $(3\times2)$ $ C F = \left[\begin{array}{rr}{2} & {0} \\ {1} & {4} \\ \color{gray}{5} & \color{gray}{-1}\end{array}\right] \left[\begin{array}{rr}{2} & \color{#DF0030}{-1} \\ {4} & \color{#DF0030}{-1}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ C$ , with the corresponding elements in column $j$ of the second matrix, $ F$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ C$ with the first element in ${\text{column }1}$ of $ F$ , then multiply the second element in ${\text{row }1}$ of $ C$ with the second element in ${\text{column }1}$ of $ F$ , and so on. Add the products together. $ \left[\begin{array}{rr}{2}\cdot{2}+{0}\cdot{4} & ? \\ ? & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ C$ with the corresponding elements in ${\text{column }1}$ of $ F$ and add the products together. $ \left[\begin{array}{rr}{2}\cdot{2}+{0}\cdot{4} & ? \\ {1}\cdot{2}+{4}\cdot{4} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ C$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ F$ and add the products together. $ \left[\begin{array}{rr}{2}\cdot{2}+{0}\cdot{4} & {2}\cdot\color{#DF0030}{-1}+{0}\cdot\color{#DF0030}{-1} \\ {1}\cdot{2}+{4}\cdot{4} & ? \\ ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{2}\cdot{2}+{0}\cdot{4} & {2}\cdot\color{#DF0030}{-1}+{0}\cdot\color{#DF0030}{-1} \\ {1}\cdot{2}+{4}\cdot{4} & {1}\cdot\color{#DF0030}{-1}+{4}\cdot\color{#DF0030}{-1} \\ \color{gray}{5}\cdot{2}+\color{gray}{-1}\cdot{4} & \color{gray}{5}\cdot\color{#DF0030}{-1}+\color{gray}{-1}\cdot\color{#DF0030}{-1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}4 & -2 \\ 18 & -5 \\ 6 & -4\end{array}\right] $